1. Field of the Invention
The present invention relates to an in-plane switching liquid crystal display apparatus, and relates in particular to an in-plane switching liquid crystal display apparatus operating on an active matrix principle.
2. Description of the Related Art
The in-plane switching (IPS) format has come to enjoy widespread use in big screen monitors such as TVs. In the IPS format, display takes place through rotation of the liquid crystals about the molecular axes within a plane parallel to the substrate under the action of a horizontal electric field generated parallel to the substrate. With this format, the viewing angle is not dependent on the rise angle of the molecular axes, and viewing angle characteristics are therefore markedly improved over those of twisted nematic (TN) format.
While the IPS format offers advantages in terms of viewing angle characteristics, a pretwist angle must be provided in order for the direction of rotation of the liquid crystal molecules to be made uniform. Specifically, the initial alignment axes of the liquid crystal molecules must be inclined with respect to the direction of application of the horizontal electric field.
FIG. 1 is a schematic plan view illustrating the need for a pretwist angle in a conventional liquid crystal display apparatus. As shown in FIG. 1, in a liquid crystal display apparatus of the prior art, a comb-shaped common electrode 91 and pixel electrode 92 are formed on each pixel, and voltage is applied across the common electrode 91 and the pixel electrode 92 in order to generate a horizontal electric field 96 perpendicular to the direction of extension of the electrodes. The initial alignment direction 97 of the liquid crystal molecules is parallel to the direction of extension of the electrodes. When the pretwist angle of the liquid crystal molecule axes is inclined with respect to the direction perpendicular to the direction of the horizontal electric field, the initial alignment direction 97 of the liquid crystal molecules will be perpendicular to the horizontal electric field 96 in cases in which the pretwist angle is equal to 0 degrees, as depicted in FIG. 1, and a direction of rotation of the liquid crystal molecules will not be fixed. As a result, the liquid crystal molecules are able to rotate either left or right, and disclination lines of discontinuous alignment of the liquid crystal molecules occur at the boundaries of domains of mutually opposing rotation directions, causing picture quality to deteriorate.
For this reason, is necessary for the pretwist angle to be set to some value other than 0 degrees in order to make the direction of rotation of the liquid crystal molecules uniform. However, if the pretwist angle is too large, the white luminance will not be high enough for sufficient contrast to be achieved. Under these circumstances, it is typical practice to make the pretwist angle from 10 to 20 degrees.
FIG. 2 is a schematic plan view showing a pixel of a first conventional liquid crystal display apparatus. The drawing depicts the case of a single domain in which the liquid crystal molecules are oriented in a single direction within the pixel. As shown in FIG. 2, the pixel is provided with a comb-shaped common electrode 101 and pixel electrode 102; an electric field 106 is applied in a direction perpendicular to the direction of extension of the common electrode 101 and the pixel electrode 102. In order to give a pretwist angle 108, the direction of initial alignment 107 of the liquid crystal molecules 105 is inclined by the aforementioned angle with respect to the direction of extension of the common electrode 101 and the pixel electrode 102. In cases in which the rubbing technique is used, rubbing will be carried out in a direction inclined by the equivalent of the pretwist angle 108 with respect to the direction of extension of the electrodes. In FIG. 2, the direction of initial alignment of the liquid crystal molecules 105 is inclined counterclockwise by the pretwist angle 108 with respect to the direction of extension of the electrodes, and the liquid crystal molecules 105 are rotated counterclockwise by application of the electric field 106.
In the case of multi-domain alignment in which the alignment direction of the liquid crystal molecules is divided into several domains within the pixel, the direction of rotation of the liquid crystal molecules must be different in each domain. This is accomplished, for example, by varying the initial alignment direction in each domain. FIG. 3 depicts a conventional liquid crystal display apparatus in which the initial alignment direction of the liquid crystal molecules differs between two regions of a pixel. As shown in FIG. 3, the pixel is provided with a comb-shaped common electrode 111 and pixel electrode 112, an electric field 116 is applied in a direction perpendicular to the direction of extension of the common electrode 111 and the pixel electrode 112, and the display region between the electrodes is divided into sub-domains 113, 114 by differences in the state of the initial alignment of the liquid crystal molecules. Specifically, in the sub-domain 113, the initial alignment direction 117a of the liquid crystal molecules is inclined counterclockwise by the pretwist angle 118 with respect to the direction of extension of the electrodes, whereas in the sub-domain 114, the initial alignment direction 117b of the liquid crystal molecules is inclined clockwise by the pretwist angle 118 with respect to the direction of extension of the electrodes. Application of the electric field 116 causes the liquid crystal molecules 115 to rotate in the counterclockwise direction in the sub-domain 113 and in the clockwise direction in the sub-domain 114, assuming different alignment states. In this way, the liquid crystal molecules in the individual sub-domains are provided with opposite directions of rotation by the two different initial alignment directions 117a, 117b and compensate for each another, suppressing color shift in the diagonal direction. However, special techniques such as segmented rubbing or photo-alignment are required, and throughput is low while costs are high.
For this reason, the usual method is to curve the electrodes themselves instead of varying the initial alignment direction of the liquid crystals, as disclosed in Patent Document 1 (Japanese Patent No. 3120751). FIG. 4 is a plan view schematically showing the liquid crystal display apparatus disclosed in Patent Document 1. As shown in FIG. 4, the common electrode 121 and the pixel electrode 122 are curved in a “V” shape at the boundary of the sub-domains 123, 124 while keeping each sub-domain parallel with itself. While the initial alignment direction 127 of the liquid crystals is the same in one direction, the direction of application of the horizontal electric field 126 differs for each sub-domain, and the liquid crystal molecules 125 will therefore have mutually opposite directions of rotation. Specifically, the liquid crystal molecules 125 in the sub-domain 123 will rotate in the counterclockwise direction, whereas the liquid crystal molecules 125 in the sub-domain 124 will rotate in the clockwise direction. This prior art technique also facilitates alignment by the rubbing technique.
Patent Document 2 (Japanese Patent No. 3132483) discloses means for achieving multi-domain alignment with a pretwist angle of 0 degrees. FIG. 5 is a plan view schematically showing the configuration of the in-plane switching liquid crystal display apparatus disclosed in Patent Document 2. As shown in FIG. 5, a common electrode 131 and a pixel electrode 132 are composed of a parallel electrode portion extending in the initial alignment direction 137 of the liquid crystals, and an orthogonal electrode portion 139 extending in a direction orthogonal thereto. The pixel is composed of sub-domains within the liquid crystal layer partitioned by the parallel electrode portion and the orthogonal electrode portion 139. In FIG. 5, the pixel is divided into sub-domains 133, 134 by the orthogonal electrode portion 139 of the pixel electrode 132, and each domain is further divided into two sub-domains by the parallel electrode portion of the pixel electrode 132. With this structure, the pattern of the horizontal electric field 136 produced across the electrodes differs in alternating fashion for the individual sub-domains. In this way, the initial alignment direction 127 of the liquid crystals is parallel to the parallel electrode portion, and the pretwist angle is 0 degrees. However, the direction of rotation of the liquid crystal molecules is made uniform within each sub-domain by providing the orthogonal electrode portion 139, with the direction of rotation being mutually opposite for individual sub-domains.
In connection with the in-plane switching liquid crystal display apparatus disclosed in Patent Document 3 (WO99/45430), an embodiment is described in which portions of the common electrode and the pixel electrode provided in each pixel region are curved. For example, the common electrode and the pixel electrode are composed of a parallel electrode portion extending parallel with the picture signal line, and an inclined electrode portion inclined with respect to the picture signal line, with the inclined electrode portion formed at an end of the parallel electrode portion. The parallel electrode portion occupies most of the electrode, with the inclined electrode portion being part of the whole. With this electrode structure, the pixel region between the common electrode and the pixel electrode is divided into a first sub-domain corresponding to the parallel electrode portions and a second sub-domain corresponding to the inclined electrode portions, with the electric field direction differing between the two sub-domains. It is disclosed that the initial alignment direction of the liquid crystal molecules is a prescribed direction that is shared by the two sub-domains, with the angle thereof being 15 degrees with respect to the direction of extension of the parallel electrode portion, requiring a pre-tilt angle that is not 0 degrees.
However, the prior art discussed above has a number of problems such as the following.
In the prior art illustrated in FIGS. 2 to 4, it is essential that the initial alignment direction of the liquid crystal molecules be inclined with respect to the direction of extension of the comb-shaped electrodes. In cases in which the comb-shaped electrodes are formed from a metal film, a difference in level produced by the metal film will become a problem. At a minimum, this difference in level is approximately several thousand angstroms. In the case of the rubbing technique, which represents the most common aligning means, deviation of the alignment direction occurs in proximity to areas of level difference, causing light leakage in the black state. A resultant problem is that contrast is lower.
In the prior art disclosed in Patent Document 2, while the initial alignment direction is parallel to the parallel electrode portion which lies in the direction of extension of the comb-shaped electrodes, it is perpendicular to the orthogonal electrode portion 139. Accordingly, with the rubbing technique there is the problem of a possible increase in light leakage, and further disadvantages in terms of contrast.
In the prior art disclosed in Patent Document 3, a structure is described in which part of the electrode is curved in similar fashion to the invention herein. However, the initial alignment direction is inclined with respect to the direction of extension of the parallel electrode portion, creating the problem of lower contrast with the rubbing technique, similar to that discussed above.
In cases in which the material of the comb-shaped electrodes is a transparent conductive film such as Indium Tin Oxide (ITO) rather than a metal film, the level difference produced by the film is approximately 400 Å, and the level difference is therefore rather small. However, requirements for high contrast have become very stringent in recent years, to the point that the slight leakage of light produced by a small level difference can no longer be ignored.